This invention relates to a thyristor controlled power supplying apparatus, and more particularly to a type thereof including a device for displaying an ignition phase angle and a conduction completion phase angle of thyristors, the ignitions of which are controlled equiangularly for controlling the current supplied from a power source to a load through the thyristors.
Various power supplying apparatus are known in the art wherein a number of thyristors are phase controlled for controlling the load current. In these apparatus, when each of the thyristors is in a conductive state throughout one-half cycle, that is when the ignition phase angle is equal to zero, the ignition phase angle cannot be advanced any more than the conduction completion phase angle of the immediately preceding half cycle (such a condition is hereinafter termed fully conductive state).
Furthermore, while a thyristor in the fully conductive state has an ample margin of 180.degree. in retarding the ignition phase angle, a thyristor operating at an ignition phase angle of nearby 180.degree. has almost no margin for a further retard of the ignition phase angle.
Accordingly, for an appropriate control of the ignition phase angle of the equiangularly operated thyristors, it is essential to know an actual ignition phase angle and a conduction completion phase angle all the time.